1. Field of the Invention
The present invention relates to a dental material and, particularly, to a cationically curable composition that can be favorably used as a filling/restorative material for dental use.
2. Description of the Related Art
A tooth that is damaged due to decaying or breakage is usually restored by using a photo-curable composite filling/restorative material which is called composite resin owing to its easy operation and a high degree of aesthetic appearance. This composite resin usually comprises a polymerizable monomer, a filler and a polymerization initiator. As the polymerizable monomer, there has been used a (meth)acrylate type radically polymerizable monomer owing to its favorable photo-polymerizing property.
In the dental therapy, a high degree of aesthetic appearance is an important requirement. In many cases, in general, a dental composite resin is used in a manner of being filled in the oral cavity and is, then, cured. Here, if the color tone greatly changes before and after the curing, the color tone may change after curing making it difficult to obtain a favorable aesthetic appearance though the color might have been in agreement with that of the neighboring teeth at the time of being filled (before curing) (if ΔE* that represents the degree of discoloration is not larger than 1.5, it has been said that the discoloration cannot usually be perceived by naked eyes). In addition to aesthetic appearance immediately after the therapy, it is further requested that the aesthetic appearance does not wane even after the use. For instance, it is necessary to avoid such an occurrence that the dental material used in the oral cavity easily undergoes the discoloration or is colored due to various coloring substances stemming from the foods.
However, the radically polymerizable monomer is hindered by oxygen from being polymerized. When the radically polymerizable monomer is polymerized and cured in the oral cavity, therefore, an unpolymerized layer or a layer polymerized to a low degree remains on the surface posing a problem in that the unpolymerized layer is colored/discolored with the passage of time making it difficult to obtain a satisfactory aesthetic appearance. After the curing, therefore, the unpolymerized layer must be removed by polishing.
Besides, the radically polymerizable monomers are usually of the type of addition polymerization leaving a problem of an essentially large coefficient of contraction.
To polymerize and cure the filling/restorative material such as the composite resin after filled in the cavity of a tooth that must be restored, the filling/restorative material that is filled must be irradiated with light and the polymerization starts from the surface thereof, i.e., from a position remote from the dentin. Due to the contraction by polymerization, however, stress is produced causing the filling/restorative material to exfoliate from the dentin, and a gap tends to develop between the tooth and the filler/restorative material. To cope with the stress produced by the contraction by polymerization, there have been proposed adhesives or dental use that produce very strong adhesion. However, the state of the tooth varies depending upon the individuals or depending upon the teeth even in the same person. Even by using such dental adhesives, therefore, perfect adhesion is not, in practice, obtained for all teeth. Therefore, it has also been desired to develop a dental composite resin which contracts in amounts as small as possible when polymerized and does not develop a gap despite the adhering force may not be very large depending upon the condition of the tooth. To obtain a large adhering force, further, the above adhesive requires a complex technology boosting the cost. Therefore, it has been desired to simplify the use.
As the polymerizable monomers of which the polymerization is not hindered by oxygen, there can be exemplified alkenyl ether compounds as represented by vinyl ether and 1-propenyl ether, as well as cationically polymerizable monomers such as epoxide and oxetane which are cyclic ether compounds (prior art 1 appearing later).
Among them, alkenyl ethers undergo the polymerization and curing very quickly accompanied, however, by a problem of being colored after curing. Like the (meth)acrylate compounds, further, the alkenyl ethers are vinyl monomers of the addition polymerization type and undergo the contraction by polymerization to a large extent.
On the other hand, the epoxy compounds and the oxetane compounds are the ring opening-polymerizable compounds which, after curing, are colored very little compared to the alkenyl ethers and, besides, are contracted considerably little by polymerization. It has therefore been proposed to use these components for dental curable compositions that must satisfy the requirements of a high degree of aesthetic appearance and a small contraction by polymerization as described above (see, for example, prior arts 2 to 7 appearing later).
However, the epoxy functional group polymerizes slowly as compared with the alkenyl ether functional group, and is not suited for the dental applications where quick polymerization and curing are required in the oral cavity.
On the other hand, the oxetane functional group has been regarded to be polymerized quicker than the epoxy functional group. Like other cationically polymerizable monomers, however, the oxetane functional group essentially has a problem of subject to be hindered by water from being polymerized. When used in the oral cavity, therefore, the oxetane functional group tends to form an unpolymerized layer on the surface still leaving room for improvement.
Study has been forwarded extensively concerning the cationically polymerizable monomers even in the fields other than the dental use, and there have been proposed a technology that uses an oxetane compound of a special structure (e.g., prior art 8) and a technology that uses a particular polymerization initiator (e.g., prior art 9) in order to solve the above problem in that the polymerization is hindered by water.
Prior Art 1: Radotech Academy, “Present Status of UV•EV Curing Technology and its Prospect”, CMC Publishing Co., Dec. 27, 2002, pp 45-48.                Prior Art 2: JP-T-10-508067        Prior Art 3: JP-A-11-130945        Prior Art 4: JP-T-2001-520758        Prior Art 5: JP-T-2001-520759        Prior Art 6: JP-A-8-245783        Prior Art 7: JP-A-2004-149587        Prior Art 8: JP-A-2004-91553        Prior Art 9: JP-A-2004-91698        